From the standpoint of environmental protection, there is an urgent need for CO2 emission reduction in steel production processes. To reduce CO2 emission in steel production processes, several measures are being studied and undertaken, including increasing the amount of cold iron source such as scrap iron, used as an iron source to reduce the blend ratio of hot metal. The reason for an increase in the amount of cold iron source is that in the production of steel products, although vast amounts of energy are required to reduce and melt iron ore and large amounts of CO2 are emitted in the production of hot metal (molten iron) in a blast furnace, cold iron sources only require heat for melting and, thus, the amount of cold iron source used in steel making processes can be increased to reduce energy consumption and CO2 emission. The term “molten iron” as used herein refers to a molten iron source and broadly includes hot metal produced in a blast furnace, molten steel produced from scrap iron in an electric furnace, and molten steel produced by decarburization refining of hot metal, for example.
In a steel production process including a blast furnace and a converter in combination, heat sources that melt a cold iron source mainly include the sensible heat of hot metal and the heat of combustion of carbon and silicon in the hot metal. As such, a large amount of cold iron source cannot be melted in such a steel production process intrinsically. Further, a processing step of preliminary dephosphorization of hot metal has recently been available, and the temperature of the hot metal decreases due to this additional processing step. Furthermore, decrease in the carbon and silicon concentrations of hot metal due to oxidation in preliminary dephosphorization is an unfavorable factor in melting the cold iron source. The preliminary dephosphorization of hot metal is a process of removing phosphorus in the hot metal to a certain extent by preliminarily performing dephosphorization of the hot metal before decarburization refining in a converter. In a steel production process in an ironworks, not only hot metal but also molten steel produced by decarburization of hot metal, for example, is subjected to oxidation refining such as dephosphorization.
In preliminary dephosphorization, decarburization refining in a converter, and oxidation refining in a reaction vessel, therefore, various methods of increasing the heat margin of molten iron such as hot metal or molten steel have been proposed. For example, Japanese Unexamined Patent Application Publication No. 9-20913 proposed a method of adding a carbon source to slag generated during preliminary dephosphorization and blowing an oxygen source into the slag to burn the carbon source, thereby depositing the heat of combustion of the carbon source on the hot metal.
Japanese Unexamined Patent Application Publication No. 60-67610 discloses a method of burning carbon monoxide (CO) produced in a converter with oxygen blown through a lance above a molten hot iron bath surface (secondary combustion) and depositing the heat of combustion on the molten iron.
Dephosphorization of hot metal has a problem of fluorine leached out from slag generated in the dephosphorization. Thus, there is a demand for efficient dephosphorization without using a fluorination source such as fluorite.
To this end, Japanese Unexamined Patent Application Publication No. 2000-345226 discloses a method of dephosphorization in a top and bottom blown converter, comprising blowing a mixed powder of CaO and Al2O3 over hot metal through a top-blowing lance while stirring the hot metal by blowing an agitation gas through the bottom.
Japanese Unexamined Patent Application Publication No. 11-080825 discloses a method of promoting the slagging of flux to improve the metallurgical reaction characteristics of dephosphorization, for example. In that method, oxygen gas, as well as a fuel gas such as natural gas, and a flux such as calcium oxide, are supplied to hot metal through a top-blowing lance, thereby the flux passing through a flame of the fuel gas can be supplied in a molten state to the hot metal.
Japanese Unexamined Patent Application Publication No. 2005-336586 and Japanese Unexamined Patent Application Publication No. 2007-92158 disclose a method of providing a top-blowing lance with function of a burner, ejecting a dephosphorization agent through a central hole of the burner, thereby the dephosphorization agent is heated and added to slag to promote slagging to improve the efficiency of a metallurgical reaction.
However, the related art has the following problems. The addition of a carbon source to slag in Japanese Unexamined Patent Application Publication No. 9-20913 increases the temperature of hot metal, but causes contamination with sulfur contained in the carbon source and increases the sulfur concentration of the steel. Having a sufficient combustion time of the carbon source results in a long refining time and increased production costs. Furthermore, combustion of the carbon source increases CO2 emission.
To transport molten iron such as hot metal to a converter or the like, the molten iron may be contained in a reaction vessel such as a torpedo car or a ladle, that has a smaller freeboard than the converter. On the assumption that decarburization is performed, converters generally have a relatively large freeboard (2.0 to 5.0 m) in consideration of splashing during decarburization. On the other hand, the reaction vessel has a freeboard of 0.8 to 2.2 m. In actual operation in an ironworks, oxidation refining such as dephosphorization of molten iron is performed even in a reaction vessel having a relatively small freeboard. Japanese Unexamined Patent Application Publication No. 9-20913 does not give due consideration to the positional relationship between the reaction vessel and a top-blowing lance in such a case.
In the method disclosed in Japanese Unexamined Patent Application Publication No. 60-67610, CO generated in the converter and oxygen blown through the lance burn over the hot metal bath surface and heavily damages the refractory of the furnace body.
In the method disclosed in Japanese Unexamined Patent Application Publication No. 2000-345226, although added Al2O3 reduces the melting point of CaO and promotes slagging of CaO, an increased Al2O3 concentration of the slag results in damage to the refractory of the furnace body, which brings concern about increase in cost, and decrease in the dephosphorization rate.
In oxidation refining in a converter using the top-blowing lance in Japanese Unexamined Patent Application Publication No. 11-080825, the top-blowing lance height may vary during blowing. Variations in top-blowing lance height result in large differences between the flame length and the lance height. Thus, a powdery smelting agent may be ineffectively heated.
Japanese Unexamined Patent Application Publication No. 11-080825 relates to blowing in a converter using the top-blowing lance and does not relate to blowing in a reaction vessel having a relatively small freeboard. In oxidation refining, for example, dephosphorization of molten iron, since a reaction vessel has a smaller freeboard than a converter, the powdery smelting agent may be ineffectively heated with a burner flame in Japanese Unexamined Patent Application Publication No. 11-080825. It is conceivable that combustion is not completed within the lance height, unburned fuel gas reaches the molten iron bath surface, and combustion gas undergoes a decomposition reaction. This decomposition reaction is an endothermic reaction and generally a very unfavorable factor for oxidation refining such as dephosphorization in terms of the deposition of heat on the powdery smelting agent.
In the methods disclosed in Japanese Unexamined Patent Application Publication No. 11-080825, Japanese Unexamined Patent Application Publication No. 2005-336586 and Japanese Unexamined Patent Application Publication No. 2007-92158, a dephosphorization agent is transported with oxygen gas through a top-blowing lance having a quadruple or quintuple tube structure. Thus, a reactive powder, for example, containing pure iron cannot be blown through the top-blowing lance. Because of a low blowing rate and a low propane flow rate, it is not expected that the hot metal blend ratio is sufficiently reduced.
It could therefore be helpful to provide a top-blowing lance that can efficiently provide (deposit) the heat of combustion of a burner to molten iron without failures as described above and can advantageously reduce the hot metal blend ratio, and a method of refining molten iron and a smelting reduction method for molten metal using the blowing lance.
It could also be helpful to provide a method of refining molten iron including oxidation refining such as preliminary dephosphorization or decarburization refining of hot metal, the method including using a top-blowing lance having a burner function to form a burner flame at the lower end of the top-blowing lance, and while heating a powdery smelting agent with the flame by using the top-blowing lance, blowing the powdery smelting agent over the molten iron, to which a cold iron source has been added, in a converter, wherein the powdery smelting agent can be efficiently heated and the blend ratio of the cold iron source in the hot metal can be consistently increased.
It could therefore be helpful to provide a method of refining molten iron including oxidation refining such as dephosphorization of the molten iron, the method including using a top-blowing lance having a burner function to form a burner flame at the lower end of the top-blowing lance, and while heating a powdery smelting agent with the flame by using the top-blowing lance, blowing the powdery smelting agent over the molten iron in a reaction vessel having a relatively small freeboard, wherein the powdery smelting agent can be efficiently heated and the heat margin of the molten iron in the reaction vessel can be increased.